An accuracy diagnostic device includes a change amount detecting unit, a change amount recording unit, an accuracy change predictor, and an accuracy adjustment timing presenting unit. The change amount detecting unit detects a magnitude of a change in at least one of a state of a machine tool and a surrounding environment of the machine tool as a change amount. The change amount recording unit records the change amount. The accuracy change predictor predicts a future accuracy change of the machine tool using the change amount recorded in the change amount recording unit. The accuracy adjustment timing presenting unit presents a timing at which an accuracy adjustment is to be required for the machine tool based on the accuracy change predicted by the accuracy change predictor.